Carbohydrate loading is the practice of systematically increasing carbohydrates during the taper phase of training (see also: doubling down on portions of pasta) to ensure glycogen stores are adequately high before a key event. The goal is to delay fatigue resulting from low glycogen as long as possible.
First things first…
How do you carbohydrate load?
Protocols and practices have evolved over the decades with respect to dosing of carbohydrates, length of protocol, and even whether there was an initial period of reduced carbohydrate intake.
Modern protocols are much shorter than they were historically. They are now only about 36-48 hours in duration with some suggestions that even as little as 24 hours is sufficient.
But let’s get specific: Dosing of carbohydrate intake is 10-12g/kg of bodyweight (4.5-5.5g/lb) during this time. For someone weighing 82kg (180lbs), this makes for a feast. In April, David Lipman, Supersapiens' Director of Applied Science and Content, ran the 2022 Boston Marathon. This is an example of his carbohydrate load.
A potential reason that shorter protocols may be effective is that the body will shift some of its substrate use based on diet contents. That is, higher carbohydrate diets shift metabolism to burn a little more carbohydrates for exercise and vice versa. In a recent paper, researchers showed that even one high-fat meal pre-exercise may help limit use of glycogen after carbohydrate loading. Another paper sought to elicit the performance benefits of a high-fat meal versus a high-carb meal pre-exercise. In this case, 4 hours prior or after a period of carbohydrate loading.
Next, they compared these two scenarios, adding either placebo jelly or maltodextrin jelly. That provided four conditions in the study: high-fat or high-carb meal and a placebo or maltodextrin jelly. The practice of eating and then having some carbohydrate just before the start of an effort reflects a regular practice for endurance athletes. (Gel on the start line, anyone?) The result: the carbohydrate loading protocol, followed by a high-fat meal 4 hours prior to exercise, and a maltodextrin jelly was seen as improving performance the most of all scenarios in runners.
That said, it is too early in the research phase to recommend these practices. It may yield no real benefit, but these do serve as a good example of the impact of recent diet on substrate use and the interest in the area.
It is generally suggested that your diet during a carbohydrate loading protocol be low in fiber content, to both aid in the volume of intake and limit potential gastrointestinal issues during the event. This may be part of the reason for athletes often drinking a lot of carbohydrates during carbohydrate loading protocols rather than just eating them.
How do you use CGM for carbohydrate loading?
As covered in this recent article, there is a relationship between glucose and glycogen, but it is non-linear. You can increase stability via efforts like changing food order. That said, many athletes find an increase in their average glucose as a result of carbohydrate loading, as mentioned here. Some athletes use their glucose exposure (average glucose each hour multiplied by hours of the day passed) and daily average glucose to ensure they are fueling enough. The relationship between glycogen stores and glucose levels may be, in part, a reflection of the body’s metabolic state, that of anabolism or catabolism – building up or breaking down respectively.
With an increased carbohydrate intake, it is most likely that glucose will be less stable. This is a result of an increased number of both rushes and rises in glucose and their resulting returns to baseline (or below). A second factor contributing to this is a reduced training volume during taper.
What does carbohydrate loading with Supersapiens look like?
Back to David Lipman, our Director of Applied Science and Content, and his experience in the Berlin Marathon: Here is a collection of his data taken from the Supersapiens dashboard from the days leading into the race and race day.
Glucose stability and carbohydrate loading
You can see that as David increased his carbohydrate intake, his glucose became less stable. (The higher the number, the less stable.) Specifically, he started increasing intake drastically on Friday leading into the Sunday race.
Glucose rushes and carbohydrate loading
In similar fashion, rush count increased steadily from a baseline prior to Friday and was highest on race day. Almost all rushes in the days prior to the race (25-09) were food related, whereas on race day, there were some mood and movement related rushes.
Daily glucose during carbohydrate loading
Both glucose stability and average are much higher than David’s baseline here, which is to be expected given an increase in carbohydrate intake. (Specifically, lower fiber carbohydrates and somewhat limiting intake of fat and protein) Additionally Friday was a day off training, also likely contributing to the higher and less stable glucose. Of note, sleep glucose follows the pattern described in this recent blog post.
Despite fewer food events made or at least seemingly so, this day had a significant amount of intake. Specifically, David drank a lot of carbohydrates to ensure he could manage his goal of 800g of carbohydrates for the day. These were sipped throughout the day rather than having a discrete bolus like a meal.
For context, David’s normal daily average glucose is around 100-105mg/dL or so. That means the averages on these two days of carbohydrate loading represent a significant increase from baseline, which was present prior to the carbohydrate loading and following the race.
Supersapiens data for marathon running after carbohydrate loading
You can see David woke up early to have breakfast 4 hours prior to the race start. From there, you will also notice some mood (stress) related rushes on the way to the race start. Post race, there is a large rush, likely to do with a mixture of mood, fuel taken later in the race and the reduction in intensity. This rush is followed by a significant drop, possibly related to insulin given the prior rush or significant glycogen depletion (or a combination of the two).
Using the Training Peaks integration with the Supersapiens Dashboard, you can see David’s glucose data overlaid on top of his pace, heart rate, and elevation data. After the initial surge of both pace and glucose from the start line, there is a slowly increasing glucose. This likely reflects two things. The first: David’s glycogen levels still being high, in addition to a pace which, at the time, was very easy. The second: his nutrition strategy (75g of carbohydrates/hr starting 10mins into the race). David’s pacing was very even throughout the race, but the only mild hill in the race means that his glucose decreases (likely through increased utilization) from around 10:40am to 11:30am.
David’s glucose score was 70, penalized for 3 drops (the initial return down from the starting surge and the return from the high levels early in the race, in addition to the unexplained drop very late in the event), some time above his Glucose Performance Zone (GPZ) and a negative slope. So while he raced really well and ran a significant personal best, there may be room for improvement in his fueling, specifically with respect to the late drop. That said, with an average glucose of 148mg/dL and a stability of 15, he clearly had good circulating glucose availability for the bulk of the race.
To aid in answering the question of whether there was adequate carbohydrate intake during the exercise events, the science team at Supersapiens has been working on a new way of evaluating glucose behavior in response to work done (HR or power). This is currently in late stage testing under the working title of 'Glucose Effectiveness'. David's Glucose effectiveness from the race can be seen below, in addition to his running power, elevation of the course, pace and heart rate. For running it is best to use heart rate for glucose effectiveness though the power relationships is there also. Values between 0.5 and -0.5 are considered to be within the realms of appropriate glucose response do need and such David's fueling for this marathon was appropriate, aligning with his performance and feeling.
How Much Carbohydrate and Fat Are Burned in a Marathon?
Back to the studies mentioned in the earlier in this blog and the metabolic shifts favoring carbohydrate or fat utilization.
Below is some other early phase work from the science team at Supersapiens looking at substrate (carbohydrate and fat) utilization based on intensity and glucose. Again, using David's Berlin Marathon data, you can get an appreciation for what is burned and where the above research may help.
This data, whilst still not perfect, gives good insight into the heavy carbohydrate utilization that occurs in marathon running. This is the reason for both the need to carbohydrate load and the fueling strategy David utilized during the marathon.
For more on building a marathon fueling plan listen to our podcast on the topic.
The Take-Home Message:
- Carbohydrate loading protocols will likely mean your glucose is less stable and may increase your average. Many athletes have found this average to be by about 5-10mg/dL. (David’s protocol yielded an increase of just over 10mg/dL in this case)
- There are instances where increased rushes, less stable glucose, and higher average are the goal. For instance: carbohydrate loading.
- Interpreting your glucose score needs context; understanding whether something like a drop is expected or not is key to using the feedback given by the glucose score to improve your fueling.
- Insights to alter your fueling plan should take into account a bigger picture than just the event glucose; they should include the prior few days and time post event to give an indication of both areas for optimization and drivers of glucose during the event.
For more insights into marathons with Supersapiens, check out this article on top tips for marathons or listen to our podcast episode on marathon fueling. Or if you want to read about the first time glucose was measured in the Boston marathon, check out this article and its follow-up a year later.
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